Power lines of most electrical power supply systems transmit electrical power of alternating current (AC) at a chosen operating voltage (e.g., 110 V, 120 V or 220 V AC) and operating frequency (usually 50 Hz or 60 Hz) via one or more phases (e.g., single-phase, two-phase or three-phase configuration). Amongst several methods which have been devised for controlling output-type electrical apparatuses connected to a given electrical power line, the simplest technique involves the use of an input device, such as on/off switch, to be connected directly to the output-type electrical device, e.g., a light bulb or a heater. However, this method suffers from the obvious problems of being very cumbersome and requiring manual control.
Another known technique for controlling electrical apparatuses connected to a power line is an indirect control method, which involves the use of control systems such as programmable logic controllers (PLC) or distributed control systems (DCS), where all inputs and outputs are isolated from each other. Due to the cost associated with the above-mentioned control systems, the indirect control method has been used almost exclusively in industrial applications.
Yet another technique for controlling electrical apparatuses connected to a power line involves transmitting electrical control signals over the given electrical power line. Most existing methods of transmitting electrical control signals over electrical power lines utilize high frequency (i.e., frequencies at least two orders of magnitude higher than the power line frequencies of 50 Hz/60 Hz or 200 Hz/400 Hz) transmission for communication between two ports physically connected to the power line. Typically, ultrasonic frequencies (in the case of office telephones), amplitude modulated (AM) radio frequencies (RF), or frequency modulated (FM) radio frequencies are utilized in order to reduce line noises and increase sensitivity and selectivity.
All control methods which are based on radio frequency (RF) communication experience several problems. First, RF communication methods are very sensitive to electrical noises, especially when an electrical system has any reactive (capacitive or inductive) load. Second, RF communication generates so-called "RF pollution," which can be detected outside of the system. Third, RF signals penetrate other electrical systems that are connected to the same power source, leaving the control system vulnerable to detection and manipulation by intruders. In addition, RF systems are bulky, expensive, employ many electronic components and require FCC approval for use. Finally, all devices which are sought to be controlled by the RF system must be plugged into the control apparatus; there is no RF-control product on the market suitable for retrofit applications.
A somewhat different method of transmitting electrical control signals over electrical power lines utilizes the principle of sending a control signal to an electrical device connected to a power line during a period of time when the sine-wave of the AC power line voltage passes the zero potential, i.e., when a "zero crossing" occurs. By way of example, for a 60 Hz power line zero crossings occur 120 times per second, or once every 8.3 milliseconds. An output-controlling signal is generated by a voltage pulse created from a discharge of a capacitor that has been charged to one-half of the peak-to-peak voltage value of the power line in the preceding half of the AC sine-wave cycle. These pulses can then be detected along the length of the power line if the pulses have sufficient magnitude to compensate for the attenuation. This method of sending a voltage pulse generated from a capacitor to control electrical devices coupled to a power line is described in U.S. Pat. No. 4,328,482 issued to Paul Belcher and Daniel Hobel, entitled "REMOTE AC POWER CONTROL WITH CONTROL PULSES AT THE ZERO CROSSING OF THE AC WAVE."
The method described in U.S. Pat. No. 4,328,482 suffers from several shortcomings. First, because the pulse-generating capacitor is directly dependent on the line voltage for the charging of energy, the pulse-generating capacitor must be nonpolarized and the capacitor's charging cycle is entirely dependent on the particular AC frequency utilized on the power line. For example, a capacitor charged by a power line utilizing a 60 Hz frequency would require a quarter of an AC sine-wave cycle, or approximately 4.16 msec., to be charged to one-half of the peak-to-peak voltage value of the power line. The device described in U.S. Pat. No. 4,328,482 utilizes a triac, which is triggered at each positive and negative peak of the AC line voltage, to charge the pulse-generating capacitor, as shown in FIG. 5. Subsequent to each positive and negative peak of the AC line voltage, the triac is again triggered at the immediately-following zero crossing to cause the pulse-generating capacitor to discharge. The waveform for the AC line voltage associated with the method and device described in U.S. Pat. No. 4,328,482 is shown in FIG. 3.
Because of the dependence of the charging cycle on the AC line frequency, the prior art method and device of U.S. Pat. No. 4,328,482 are limited to providing one voltage pulse per every zero crossing for each capacitor utilized. Similarly, even if one desires to transmit control pulses at points other than zero-crossing points of the waveform of the AC power line voltage, only one such control pulse may be generated per each half cycle of the waveform of the AC power line voltage. In order to provide multiple pulses, multiple capacitors must be utilized, which requirement increases the size, the number of components and the cost of the control device.
Another drawback of the prior art control device disclosed in U.S. Pat. No. 4,328,482 is that the pulse-generating capacitor must have a sufficiently large capacitance in order to generate voltage pulses having magnitudes adequate to compensate for the pulse attenuation, which varies as a function of the length of the power line. It should be readily apparent to those skilled in the art that the energy stored in a capacitor is equal to one half of the product of the capacitor capacitance and the square of the charging voltage across the capacitor. Because the maximum charging voltage of the pulse-generating capacitor in the prior art device of U.S. Pat. No. 4,328,482 is directly dependent on the AC line voltage, the only way to increase the amplitude of the discharge pulse generated by the capacitor in order to compensate for the attenuation of the pulse is to increase the capacitance value of the capacitor. Increase in capacitance translates into increase in capacitor size and cost. For example, the prior art method and device of U.S. Pat. No. 4,328,482 would require the use of a big, expensive, non-polarized capacitor in order to produce adequate voltage pulses which compensate for the attenuation of the pulses. Furthermore, the prior art control device of U.S. Pat. No. 4,328,482 is unable to generate a pulse having a magnitude greater than one-half of the peak-to-peak voltage of the AC power line.
Yet another drawback of the prior art method and device of U.S. Pat. No. 4,328,482 is that a significant power factor correction is required in order to reduce harmonic distortion of the pulse-generating, control apparatus connected to the AC power line. Power factor refers to the ratio of real (or average) power to apparent power. The power factor will reach its maximum value, unity, when the voltage and current are in phase. This situation exists when a circuit is purely resistive. For a circuit that is not purely resistive, unity power factor can also be achieved for specific element values and a specific frequency.
In the method and device described in U.S. Pat. No. 4,328,482, the pulse-generating capacitor is charged to one half of the peak-to-peak AC line voltage directly from the AC power line when the breakdown device (triac) connected in series to the pulse-generating capacitor is triggered at the positive or negative peak of the AC line voltage waveform. Since the power is taken near the AC line voltage peak, the resulting charge-current spike is very nonsinusoidal with a high content of harmonics. This situation results in a low power factor condition in which the apparent power is significantly higher than the real power.
There is therefore a need for an improved system and a method for controlling electrical devices connected to an AC power line by means of control signals transmitted via the AC power line.